Research On The Cold Air Dynamic Filed For A 350 MWe Supercritical Down-Fired Boiler

With Low quality coal mining with low volatile matter increasing, down-fired boilers are developping fast as a kind of boiler that is fit for burning low quality coal in China. However, down-fired boilers are all built using foreign technolgy in China. There is lack of a kind of down-fired combustion technolgy with Chinese independent property rights. On the basis of digestion and absorption towards foreign down-fired combustion technolgy, multi-injection multi-stage combustion technolgy is invented as a new down-fired combustion technolgy.This paper researches on a 350MW down-fired boiler using muti-injection multi-stage combustion technolgy. A 1:10 model in accordance with Similarity Theory was built. Cold aerodynamics field in the furnace was analyzed on this model.In aerodynamics field tracing experiment, aerodynamics field is showed under the operation parameters in BMCR condition of the down-fired boiler. The result showed cold aerodynamics field in the furnace was steady. The jet was flowing downwards with a good rigidity and could reach the upper and middle region of hopper. Aerodynamics field showed a symmetric W-shaped field. OFA injection could reach middle region of the furnace. OFA injection had little influence on aerodynamics field in the lower part of furnace.Aerodynamics field was measured at different primary air and vent air distribution by using IFA300 constant temperature anemometer. It was found that the air flows decayed at a similar tend and penetrating distance was almost same in a range of 0.50-0.57 at different primary air and vent air distribution. When primary air momentum was smaller than inner secondary air momentum, primary air and inner secondary air put off mixing with primary air volume increasing. The length of recirculating region became smaller. When primary air momentum was larger than inner secondary air momentum, the mixing between primary air and inner secondary air decayed obviously. The length of recirculating region increased. The optimal ratio between primary air and vent air was 5:5.When secondary air ratio under arch was 5%, 15%, 24.16% and 35%, the air flow decayed at a linear velocity before it reached the secondary air region under arch. Penetrating distance decreased with secondary air ratio under arch increasing. When secondary air ratio under arch was 35%, penetrating distance in front wall region was 0.449 and the one in rear wall was 0.594, which mean aerodynamics field in the lower part of furnace deflected obviously. Secondary air under arch turned upwards after it entered front wall region of the furnace. Then it flowed towards rear wall region under arch. The jet flowed downward close to rear wall under the extrusion of air flow of front wall region. When it reached the hopper region, it kept flowing downward close to hopper wall. It turned upward at middle region of hopper and flowed toward secondary air region under arch, lifting secondary air under arch in front wall region. So considering a suitable penetrating distance and symmetric aerodynamics field, the optimal secondary air ratio under arch was 24.16%.